1. Field of the Invention
The present invention relates to light emitting diodes (LEDs). More particularly, this invention relates to light emitting LEDs having active layers that produce light at one wavelength, but that emit light at another wavelength.
2. Discussion of the Related Art
Light emitting diodes (“LEDs”) are well-known semiconductor devices that convert electrical current into light. An LED produces light by exciting electrons across the band gap between a conduction band and a valence band of a semiconductive active (light-emitting) layer. That electron transition generates light at a wavelength (color) that depends on the band gap. Thus, the color of the light (wavelength) emitted by an LED depends on the semiconductor material(s) of the active layer.
LEDs are widely available in a range of colors, for example, red, green, blue, yellow, and orange. However, conventional LEDs are relatively monochromatic light sources. Unfortunately, some applications require white light, which includes all primary colors. For example, laptop computers often require white-light backlights. Usually, white light is supplied either by incandescent bulbs or by fluorescent lamps. Although inexpensive, incandescent bulbs have fairly short lifetimes and low luminous efficiency. While more efficient, fluorescent lamps also tend to have limited lifetimes. Furthermore, fluorescent lamps require relatively large, heavy and expensive support devices, such as voltage stabilizers.
A white-light LED source could be made by fabricating closely spaced (or otherwise light-mixed) red, green, and blue LEDs that emit light in proper proportions. However, blue LEDs have been relatively difficult to fabricate, primarily because of difficulties in fabricating high quality crystals having a suitable band gap. Despite these difficulties, blue GaN-based LEDs have recently become commercially available. This has enabled white-light LEDs to actually be fabricated by mixing green, red and blue light together.
While successful in producing white light, three-component (green, red and blue) LEDs have problems. For example, three-component LEDs will use significantly more power than a single component LED. Additionally, three-component LEDs require careful balancing of optical outputs to achieve high quality white light, a balance that is difficult to maintained over time and temperature and that requires careful and expensive fabrication. The necessity of optical balancing combined with a relatively complicated drive circuitry means that three-component LEDs are, in practice, difficult and expensive to fabricate.
Because of the forgoing problems with three-component LEDs it is would be advantageous to produce white light using only a single-element LED. Such single element white-light emitting LEDs are known. For example, FIG. 1 illustrates a prior art single-element, white-light LED 12. That LED incorporates a yttrium-aluminum garnet (YAG) phosphor. Essentially, the phosphor layer produces white light from blue light. As shown, the single element white-light LED 12 is comprised of a blue-light emitting LED chip 14 that is located on a base 15, which is inside an organic YAG phosphor 16. The YAG phosphor 16 is embedded in a dome-shaped package 17 having a hemispherical top 18. The package 17 protects the resulting LED from damage caused by static electricity, moisture, and other environmental influences. Extending from the package 17 are two leads 20 and 22. Bonding wires 24 and 26 connect the anode and cathode of the LED chip 14 to the leads 20 and 22.
Still referring to FIG. 1, when electric power is applied to the LED chip 14 via the leads 20 and 22 and the bonding wires 24 and 26, the LED chip 14 emits blue-light into the YAG phosphor 16. In response, white light is emitted from the package 17.
Thus, a key to making white-light LEDs using the method illustrated in FIG. 1 is to fabricate suitable blue-light emitting LEDs. A beneficial approach to fabricating blue-light emitting LEDs is to incorporate active layers comprised of Gallium-Nitride (GaN) and Indium to produce InGaN/GaN semiconductor layers. In fact, the energy efficiency of GaN-based white light emitting LEDs has surpassed that of incandescent lamps, and is now comparable with that of fluorescent lamps.
Despite their numerous advantages, white-light emitting LEDs similar to the one shown in FIG. 1 have problems. One set of problems relates to degradation of the bonding wires 24 and 26, the LED chip 14, and the leads 20 and 22 due to direct contact and subsequent chemical reaction with the YAG phosphor 16. Additionally, the YAG phosphor 16 can be degraded by such chemical reactions.
Another problem with white-light emitting LEDs similar to the one shown in FIG. 1 is that the hemispherical top 18 of the package 17 results in a “ring pattern” in the emitted light. Thus, the emitted light has poor luminance uniformity. The hemispherical top 18 also makes it difficult to reliably coat phosphors inside the package if such coating is required.
Another serious problem with white-light emitting LEDs similar to the one shown in FIG. 1 is that the actual production of white light does not come from the light-producing LED chip 14, which emits only blue light, but from phosphor 16 within the package 17. Thus, the package not only provides protection, it is a functional requirement. This makes it difficult to form surface mounted devices.
U.S. Pat. No. 6,337,536, by inventors Matsubara et al., which issued on Jan. 8, 2002, and which is entitled, “White color light emitting diode and neutral color light emitting diode,” discloses a white-light emitting source that uses an n-type ZnSe single crystal substrate. That substrate is doped with I, Cl, Br, Al, Ga, or In emission centers, and includes an epitaxial film active layer structure of ZnSe, ZnCdSe or ZnSeTe. The active layer emits blue or blue-green light. The emission centers convert the blue or blue-green light to yellow or orange. The blue or blue-green light and the yellow or orange light synthesize white light or a neutral color light between red and blue.
While the techniques taught in U.S. Pat. No. 6,337,536 are generally successful, they have problems. For example, U.S. Pat. No. 6,337,536 teaches a thick substrate. Therefore, the light intensity is heavily dependent on the thickness of the substrate. Furthermore, the materials used in U.S. Pat. No. 6,337,536 may not be optimal in specific applications.
Therefore, a new single-element, white-light LED would be beneficial. Particularly beneficial would be a single-element, white-light LED that reduces or eliminates bonding wire, LED chip, connector lead, and phosphor degradation. Also beneficial would be a single-element, white-light LED that does not produce a ring pattern and that improves the uniformity of emitted light. Such a single-element, white-light LED would beneficially be fabricated as an on-chip, single-element, white-light LED that does not require a package for white light emissions. A method of fabricating white-light emitting diodes without coating phosphor inside packages would be useful. Also beneficial would be a single-element, white-light LED with a light output that does not depend on the thickness of a substrate. More generally, a method of fabricating light emitting diodes using tinted thin film coatings would be beneficial.